This application claims the priority of German Patent Document No. 196 45 383.6-13, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process for operating an Otto internal-combustion engine having an internal mixture formation, the point in time and the injection quantity of the respective fuel injection being changeable as a function of the load requirement of the internal-combustion engine 1 by means of an electronic control unit and the required fuel quantity being injected at a full load or high partial load in the suction stroke and at a medium or low partial load in the compression stroke of the piston into the cylinder.
The injection of the fuel quantity required for the internal mixture formation in the case of Otto internal-combustion engines preferably takes place by means of one injector respectively per cylinder in the compression stroke of the piston, in which case the late point in time of the injection permits a charge stratification in the case of which, while the overall mixture formation is lean, in the proximity of the ignition source, a rich, easily inflammable fuel/air mixture is provided. As the result of the stratified-charge operation of the internal-combustion engine with the compression stroke injection, a comparatively low fuel consumption and a reduction of the pollutant emissions can be achieved in the medium and low partial load range of the internal-combustion engine. The maximal power output of the internal-combustion engine is lower in the stratified-charge operation than during a homogeneous mixture formation so that, at full load or at a high partial load, an operating process with a fuel injection in the suction stroke of the piston is advantageous. German Patent Document DE-OS 43 24 642 A1 therefore suggests a directly injecting internal-combustion engine which operates with a charge stratification in the low-load range and with a homogenous mixture formation in the high-load range. The known internal-combustion engine has an electronic control unit which, as a function of the momentary load requirement of the internal-combustion engine, determines the point in time of the injection and the injection quantity of the respective fuel injection by the corresponding control of an injector and also controls the point in time of the ignition.
Lower gas temperatures occur during an operation of the internal-combustion engine with a late internal mixture formation by fuel injection in the compression stroke than in the case of a fuel injection during the suction stroke and a therefore homogeneous mixture formation so that, as the operating time increases during the stratified-layer operation, deposits occur on those surfaces in the cylinders which are wetted with liquid fuel during the injection. If such deposits occur on the injector nozzle, this may lead to changes of the spray pattern and can disadvantageously change the mixture formation conditions and the combustion conditions. Furthermore, deposits on the spark plug particularly on its electrodes, cause ignition problems since, starting from a certain degree of coking, the resistance of the spark plug is reduced to such an extent that the remaining voltage offered by an ignition system will no longer be larger than the break-through voltage required for generating a spark. If ignition failures occur because of a coking of the spark plug; that is, the forming of the ignition spark and therefore the mixture ignition does not take place in the concerned working cycle of the cylinder, the fuel consumption will increase and the pollutant emission will also rise as a result of unburned hydrocarbons. In the high-load operation of the internal-combustion engine with a homogeneous mixture burning by fuel injection during the suction stroke, the deposits are prevented by far higher gas temperatures and the deposits are also removed which formed during a preceding low-load operation. The known internal-combustion engine can be operated in the partial-load range only for short periods of time which are limited by the occurrence of ignition failures because of coking of the spark plug or by a deficient mixture formation because of deposits on the injector nozzle. It therefore cannot meet the requirements occurring in practice, specifically the longer operating duration in the lower partial-load range.
It is therefore an object of the invention to provide a process for operating an Otto internal-combustion engine of the above-mentioned type having an internal mixture formation, which permits an arbitrary operating duration in the medium or low load range without disadvantageous effects on the combustion quality.
According to the invention, this object is achieved by providing an engine operating process of the kind referred to above, wherein the operation of the internal-combustion engine 1 with a compression stroke injection, as a function of the time or at least one operating parameter, the control unit 6 changes over, independently of the momentary load requirement 13, to the suction stroke injection with a simultaneous reduction of the intake air quantity and, after the conclusion of a certain free-burning interval (t.sub.Fr), switches back into the operating mode with the compression stroke injection.
The change-over, which is independent of the momentary load requirement, from the operation of the internal-combustion engine with a compression stroke injection to a suction stroke injection, corresponding to the duration of the working interval to the switching-back to the compression stroke injection, causes a short-term mixture burning at high temperatures. In this case, the deposits created by the wetting with liquid fuel are thermally decomposed and, in particular, the injector nozzle and the spark plug of each cylinder are burnt free so that their optimal function is ensured. The change-over of the point in time of the injection by the electronic control unit takes place as a function of an operating parameter of the internal-combustion engine, in which case the operating intervals with the compression stroke injection with the resulting achievable advantages with respect to the fuel consumption and the pollutant emissions are as long as possible. The change-over of the operating mode in each case takes place as late as possible but in time before a damaging influence on the combustion quality by the extent of the deposits. During the change-over to the suction stroke injection and homogeneous mixture formation, the amount of intake air is simultaneously reduced so that the load point or the power output of the internal-combustion engine is the same before and after the change-over of the points in time of the injection. By means of a slightly increased injection quantity as in the stratified-charge operation in the case of the compression stroke injection and a high excess of intake air, as a result of the throttling of the rate of flow of the intake air, the homogeneous mixture formation is possible in the case of a suction stroke injection and under stoichiometric air conditions. After the expiration of the working interval for the burning-free, in addition to shifting the point in time of the injection into the compression stroke, the flow rate of the intake air is also increased in order to switch back into the stratified-charge operation with the required .lambda.-values.
Advantageously, an input quantity is fed to the control unit which is determined from an operating parameter. The control unit compares the fed input quantity with a predeterminable parameter and, in the case of a conformity, causes the change-over of the operating mode from the compression stroke injection to the suction stroke injection. After the conclusion of the working interval for the burning-free, the switch-back takes place. The parameters may be filed in a characteristic diagram and are taken out by the control unit for the momentary load point of the internal-combustion engine.
In advantageous further developments of the invention, the input quantity is continuously generated by a timer. In this case, the parameter determines an operating interval with a compression stroke injection which is maximally possible in the present load point of the internal-combustion engine and in which the extent of the formed deposits does not influence the optimal function of the internal-combustion engine. After the expiration of the respective operating interval; that is, in the case of a conformity of the input quantity generated by the timer and the parameter, the change-over takes place to the homogeneous mixture formation and a removal of the deposits by a burning-free at high gas temperatures.
In a further advantageous development of the invention, an operating parameter of the internal-combustion engine with information on the momentary combustion quality is determined as the input quantity. An impairment of the combustion quality may occur as the result of a change of the spray pattern during the fuel injection or as the result of ignition failures because of deposits and coking on the injector nozzles or the spark plugs. Changes of the burning conditions and particularly the occurrence of ignition failures affect the quiet running of the internal-combustion engine so that a determination of an unquiet running is suitable as an operating parameter for generating the input quantity for the control unit. The quiet running of the internal-combustion engine can be determined by measuring the rotational speed on the rotating flywheel of the internal-combustion engine, in which case a change-over from the stratified-charge operation to the homogenous mixture formation takes place when a load-point-specific rotational speed is reached.
In a particularly advantageous embodiment of the invention, one electric testing voltage respectively is applied to the spark plugs projecting into the cylinders for the mixture ignition and a testing current is measured which in each case flows through the spark plug and which determines the quality of the input quantity for the control unit. After the application of a known testing voltage and measuring the current intensity of the flowing testing current, the electric resistance of the spark plug can be determined according to Ohm's law. Since the electric resistance of a spark plug clearly falls with an increasing degree of coking, the measuring of the testing current and the detection of the antiproportionally acting electric resistance of the spark plug, permit a conclusion with respect to the degree of coking. If, in the stratified-charge operation of the internal-combustion engine with a compression stroke injection, the testing current measured at the spark plug corresponds to a current intensity given by the parameter, the control unit will change over to a suction stroke injection. In the case of this control of the point in time of the change-over, it is particularly advantageous that the operating intervals with the compression stroke injection are maximally long because the optimal point in time of the change-over is in each case indicated shortly before the occurrence of ignition failures. The testing voltage is applied to the spark plug during a testing interval which, in the working play of the cylinder, is away from the point in time of the ignition at which an ignition voltage is applied to the spark plugs for triggering an ignition spark for the mixture ignition sparking over between the electrodes. Preferably, the spark plug diagnosis will take place by the application of a testing voltage and the measuring of the testing current during the exhaust stroke before the application of the ignition voltage. In this case, the testing voltage applied to the spark plugs is clearly lower than the sparkover voltage necessary for generating an ignition spark, whereby the mixture formation conditions and the combustion conditions are not influenced during the spark plug diagnosis. The spark plug diagnosis can be carried out by means of direct current or alternating current. Several admissions of voltage to the spark plug successively in a pulse-type manner are also conceivable.
The spark plug diagnosis by applying a testing voltage and a measurement of the respective flowing testing current, in view of a possibly different degree of coking, can be carried out on several spark plugs or even on each spark plug of the internal-combustion engine individually. In this case, the spark plug diagnosis takes place in the same crank angle area during the respective working play of the cylinders, whereby precise information can be obtained concerning the degree of coking of the respective spark plugs in comparison to one another and a change-over of the point in time of the injection and a subsequent switching back into the operating mode with the compression stroke injection can take place at the optimal point in time as a function of the load requirement.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.